New developments in zinc-catalyzed asymmetric hydrosilylation of ketones with PMHS

The influence of structural modifications of the diamine ligand and the ZnR₂ precursor in the [ZnR₂-diamine]-catalyzed asymmetric hydrosilylation of prochiral ketones with PMHS in aprotic medium is reported. A new diamine ligand giving up to 91% ee in the reduction of acetophenone is described. The scope of this reduction system has been investigated using variously functionalized ketones and some deactivation pathways have been identified.     

Synthesis of new NHC-rhodium and iridium complexes derived from 2,2′-diaminobiphenyl and their catalytic activities toward hydrosilylation of ketones

Four new N-heterocyclic carbene (NHC)-rhodium and iridium complexes derived from 2,2′-diaminobiphenyl have been successfully synthesized and their structures have been unambiguously characterized by X-ray diffraction. Their catalytic activities for hydrosilylation of ketones with diphenylsilane were investigated. It was found that NHC-rhodium complex (6) is the best one among the four catalysts for the reduction of ketones.     

Synthesis of novel chiral tetraaza ligands and their application in enantioselective transfer hydrogenation of ketones

Novel chiral tetraaza ligands(R)-N,N′-bis[2-(piperidin-1-yl)benzylidene]propane-l,2-diamine 6 and(S)-N-[2-(piperidin-l- yl)benzylidene]-3-{[2-(piperidin-1-yl)benzylidene]amino}-alanine sodium salt 7 have been synthesized and fully characterized by NMR,IR,MS and CD spectra.The catalytic property of the ligands was investigated in Ir-catalyzed enantioselective transfer hydrogenation of ketones.The corresponding optical active alcohols were obtained with high yields and moderate ees under mild reaction conditions.     

Cationic nitridoruthenium(VI) catalyzed hydrosilylation of ketones and aldehydes

The first example of a ruthenium nitrido compound as hydrosilylation catalyst is described, using phenylsilane as reductant. A variety of ketones and aldehydes are reduced to alcohols with good to high isolated yields. Some mechanistic insight on this new system is provided on the basis of the available experimental findings.     

Rhodium-catalysed asymmetric hydrosilylation of ketones using HETPHOX ligands

The HETPHOX ligand class was applied to the rhodium-catalysed asymmetric hydrosilylation of a range of substituted acetophenones. Enantioselective hydrosilylation of acetophenone with the tert-butyl substituted HETPHOX ligand gave (R)-phenylethanol in excellent enantioselectivity (84% ee) and in good conversion (80%). When applied to the hydrosilylation of other ketones conversions up to 93% and enantioselectivities up to 88% were observed.     

Synthesis of dialkoxydiphenylsilanes via the rhodium-catalyzed hydrosilylation of aldehydes

The commercially available rhodium(I) complex [RhCl(CO)₂]₂ (1) was shown to be an effective catalyst for the reduction of carbonyls with organosilanes under mild conditions. This study focusses on the hydrosilylation of aldehydes with diphenylsilane leading to the isolation of a series of dialkoxydiphenylsilanes with low catalytic loading of complex 1.     

Phosphines with 2-imidazolium ligands enhance the catalytic activity and selectivity of rhodium complexes for hydrosilylation reactions

Phosphines with 2-imidazolium ligands can specifically vary their physical and chemical properties by altering the attached substituents. Rhodium complexes (1b-7b) exhibited excellent catalytic activity and selectivity for hydrosilylation of olefins. The selectivity of the β-adduct clearly increased when the length of the alkyl chain bound to the imidazolium cation increased. Rhodium complex 1b in BMimPF₆ can be reused without noticeable loss of catalytic activity and selectivity.     

Synthesis of 2-methylidene-1-silacyclohexanes by intramolecular hydrosilylation

The preparation of various (hex-5-ynyl)silanes was achieved following two different synthetic approaches from readily available materials such as 4-bromobutene, 6-iodohexyne and chlorosilanes. Different reaction conditions for intramolecular hydrosilylation were tested to prepare the corresponding 2-methylidene-1-silacyclohexanes. Notably, the use of Speier’s catalyst allowed the regioselective formation of the desired products in moderate yields.     

Synthesis of new binuclear ferrocenyl compounds by hydrosilylation reactions

Ferrocenyl silanes are prepared by treatment of Grignard reagents produced from 4-chlorobutylferrocene derivatives and chlorodimethylsilane in THF. Butenylferrocenes are prepared by the elimination reaction of 4-chlorobutylalkylferrocenes by sodium tert-butoxide in DMSO. A hydrosilylation reaction between a butenyl compound and ferrocenylsilane occurred in dry toluene at room temperature in the presence of the Karstedt catalyst to produce the desired binuclear ferrocenyl compound in good to high yields. The electrochemical behavior of new ferrocenyl compounds were studied by cyclic voltammetry in CH₃CN/0.1 M LiClO₄, and the relation between the peak currents and the square root of the scan rate, showed that the redox process is diffusion-limited.     

Synthesis of pyrimidines from ketones using microwave irradiation

A simple, high yielding synthesis of pyrimidines from ketones in the presence of HMDS and formamide is described. Under microwave irradiation, heteroaromatic, aryl, aliphatic, and cyclic ketones cyclized to give pyrimidines in good yields.     

Synthesis and characterization of chiral mono N-heterocyclic carbene-substituted rhodium complexes and their catalytic properties in hydrosilylation reactions

Different chiral mono-substituted N-heterocyclic carbene complexes of rhodium were prepared, starting from [Rh(COD)Cl]₂ (COD = cyclooctadiene) by addition of free N-heterocyclic carbenes (NHC), or an in-situ deprotonation of the corresponding iminium salt. All new complexes were characterized by spectroscopy methods. In addition, the structures of chloro(η⁴-1,5-cyclooctadiene)(1,3-di-[(1R,2R,3R,5S)-2,6,6-trimethylbicyclo[3.1.1]hept-3-yl] imidazolin-2-ylidene)rhodium(I) (5a), chloro(η⁴-1,5-cyclooctadiene)(1,3-di-[(1R,2S,5R)-2-isopropyl-5-menthylcyclohex-1-yl]imidazol-2-ylidene)rhodium(I) (5b) and chloro(η⁴-1,5-cyclooctadiene)(1,3-di-[(2R,4S,5S)-2-methyl-4-phenyl-1,3-dioxacyclohex-5-yl]imidazolin-2-ylidene)rhodium(I) (5i) were analyzed by DFT-calculations. The enantioselective hydrosilylation of acetophenone, ethylpyruvate and n-propylpyruvate with diphenylsilane and hydrolysis was carried out with chiral C₂-symmetrical mono-substituted N-heterocyclic carbene rhodium complexes giving for the first time an enantioselective excess of up to 74% ee in the case of the n-propylpyruvate.     

P/S ligands derived from carbohydrates in Rh-catalyzed hydrosilylation of ketones

Reported is the synthesis of a number of diastereomerically pure cationic Rh(I)-complexes I starting from phosphinite thioglycosides. These complexes were used in the asymmetric hydrosilylation of prochiral ketones. The reactivity and enantioselectivity of the reaction was shown to be dependent on the pyranose ring, the substituent at the sulfur atom, the hydroxylic protective groups and most significantly on the alkene co-ligand.     

Furanoside thioether–phosphinite ligands for Rh-catalyzed asymmetric hydrosilylation of ketones

A series of thioether–phosphinite ligands, easily prepared in a few steps from inexpensive d-(+)-xylose, were tested in the Rh-catalyzed hydrosilylation of ketones. Systematic variation of the electronic and steric properties of the thioether moiety provided useful information about the ligand parameters which control enantiodiscrimination. The results show that the enantiomeric excesses are strongly dependent on the steric properties of the substituent on the thioether moiety and on the steric properties of the substrate. High activities and good enantiomeric excesses (up to 90%) were obtained in the hydrosilylation of several aryl ketones.     

Enantioselective transfer hydrogenation of ketones with planar chiral ruthenocene-based phosphinooxazoline ligands

1,2-Disubstituted planar chiral ruthenocene-based phosphinooxazoline ligands (Rc-PHOX, 3 and 4) were synthesized easily and applied in the transfer hydrogenation of ketones to chiral alcohols using 2-propanol as a source of hydrogen with excellent enantioselectivity and high catalytic activity.     

Synthesis of rhodium N-heterocyclic carbene complexes and their catalytic activity in the hydrosilylation of alkenes in ionic liquid medium

Rhodium complexes bearing N-heterocyclic carbene (NHC) ligands were prepared from bis(η⁴-1,5-cyclooctadiene) dichlorodirhodium and 1-alkyl-3-methylimidazolium-2-carboxylate, and the catalytic properties of rhodium complexes prepared in the hydrosilylation of alkenes in ionic liquid media were investigated. It was found that both the catalytic activity and selectivity of the rhodium complexes bearing NHC ligands were influenced by the attached substituents of the imidazolium cation. Additionally, rhodium complexes bearing NHC ligands in ionic liquid BMimPF₆ could be reused without noticeable loss of catalytic activity and selectivity.     

Bridge functionalized bis-N-heterocyclic carbene rhodium(I) complexes and their application in catalytic hydrosilylation

A series of chelating bridge functionalized bis-N-heterocyclic carbenes (NHC) complexes of rhodium (I) were prepared by reacting the corresponding imidazolium salts with [Rh(COD)Cl]₂ in an in-situ reaction. For the N-methyl substituted complex with a PF₆-anion an X-ray crystal structure was exemplary obtained. All complexes were spectroscopically characterized and tested for the hydrosilylation of acetophenone.     

Synthesis of a novel functional polymer immobilized platinum complex and its application in the catalytic hydrosilylation of 3,3,3-trifluoropropene with triethoxysilane

A novel polymer has been synthesized using 2-vinylpyridine as a functional monomer and allyl polyethylene glycol as a cross-linking agent, and platinum has been immobilized on this synthesized polymer. The resulting immobilized catalyst showed superior catalytic performance for the hydrosilylation of 3,3,3-trifluoropropene with triethoxysilane as compared to homogenous platinum catalyst, polystyrene-immobilized platinum or other hydrosilylation catalysts. The conversion of silane is about 100% and the maximum yield of β-adduct is 92.3% with slightly α-adduct. Furthermore, the catalyst showed sufficient stability that it could be reused three times without noticeable inactivation.     

Asymmetric transfer hydrogenation of ketones catalyzed by nickel complex with new PNO-type ligands

The new polydentate mixed-N,P,O chiral ligands have been synthesized by the condensation of bis（o-formylphenyl）- phenylphosphane and R-phenylglycinol in CHCl₃,and fully characterized by IR,NMR and EIMS spectra.These ligands were employed with a simple Ni complex Ni（PPh_s）₂Cl₂ in situ as catalytic systems for asymmetric transfer hydrogenation of ketones,and the corresponding optical alcohols were obtained with up to 84%ee under mild conditions.     

Homogeneous gold-catalyzed hydrosilylation of aldehydes

The catalytic hydrosilylation of aldehydes in the presence of PBu₃ modified Au(I)-complexes was investigated. In situ IR and NMR experiments have revealed that both, the ligand PBu₃ and the substrate aldehyde play an important role in stabilizing the gold catalyst and/or forming the catalytically active species. In their absence the reducing power of silane destabilizes the gold (I) catalyst giving rise to gold clusters or particles. Several side reactions involving water and oxygen were also investigated. A plausible reaction pathway as an alternative to the well-accepted mechanism for the transition-metal homogeneously catalyzed hydrosilylation of aldehydes has been proposed to accommodate the experimental observations.     

Copper-catalyzed asymmetric hydrosilylation of ketones using air and moisture stable precatalyst Cu(OAc)2·H2O

Air and moisture stable copper(II) salts can be used to catalyze the hydrosilylation of aromatic ketones. The combination of catalytic amounts of copper(II) acetate or copper(II) acetate monohydrate (Cu(OAc)₂·H₂O) and BINAP in the presence of organosilanes as the stoichiometric reducing agent generates an active catalyst for the reduction of ketones.